Process negative sigma correctly with combrule 2 or 3

The initial combination rule code (for sigma/epsilon)
did not take negative sigma rules into account, which
caused segfaults instead of the values reaching the code
in convparm.c that handles sigma<0 during conversion
to c6/c12 (where it is used to signal c6=0).

Fixes #1391.

Change-Id: I437f06d67c5ecfb58d236590288ad122bcdf2df0